Some unexpected behaviour on a Linux server could be a result of malware infection, while other malicious software might not alert to their presence. Scanning your system for different types of unwanted programs can help identify issues, or at least give you the peace of mind for having a clean server.

There are multiple options for making sure your cloud server is clean of any malware, this guide goes over a couple of scanning software you can utilise for checking your system.

ClamAV

ClamAV is a popular open source antivirus engine available on multitude of platforms including the majority of Linux distributions. Install it on your Debian system with the command below.

sudo aptitude install clamav clamav-daemon

With the required modules installed and configured, next you should update the virus database for ClamAV by running the updater application with

sudo freshclam

When you’ve finished updating the virus definitions, do a test scan to your home directory just to make sure the scanning works as it should, use the following command

sudo clamscan -r /home

Granted that your home directory didn’t contain any viruses or other type of malware, the scan should come back empty.

So how do you know it works?

For this, you can download an anti-virus test file, which is a small completely harmless program that most anti-virus software report as infected, though with an obvious test file name EICAR-AV-Test. Use the following command to download the test file to your home directory.

wget -P ~/ http://www.eicar.org/download/eicar.com

Now scan your home folder again with the same command as above, you should receive notice of one infected file at the end summary after the scan is completed. When you’ve confirmed that ClamAV finds the test file correctly, use the command below to scan it again and remove the infected file once found.

sudo clamscan --infected --remove --recursive /home

Be careful when using the –remove parameter. First run a broader scan without it, and then more localized scan when removing files, or remove them manually.

To perform a complete scan of your cloud server, use the this command

sudo clamscan --infected --recursive --exclude-dir="^/sys" /

The scan goes through each directory in your system root recursively, but skips /sys just to avoid unnecessary warning printouts, as the virtual file system consists of some unreadable files, which could not contain viruses anyway.

Rkhunter

Rkhunter is a common options for scanning your system for rootkits and general vulnerabilities. It can be easily installed using the package manager on Debian systems with

sudo aptitude install rkhunter

Once installed and before scanning you’ll need to update the file properties database with

sudo rkhunter --propupd

This lets the scanner to know the current state of certain files to prevent some false alarms. After the update, simply run the scanner with the following

sudo rkhunter --checkall

The scanner goes through some system commands, checks for actual rootkits and some malware, network and local host settings, and then gives you the summary as well as recording the findings to a log file.

Afterwards you can get a condensed look at the scan log with this command

sudo cat /var/log/rkhunter.log | grep -i warning

Go through the output to get some tips on what you could do to improve your system security.

Chkrootkit

Chkrootkit is an other popular rootkit scanner, which runs a lot of useful checks and can direct suspicions towards finding a solution. It can be installed on most distributions with the package manager, on a Debian server use the following

sudo aptitude install chkrootkit

Once done, scan your server with this command

sudo chkrootkit

The scan will check for many types of infections and print out its findings. You can scroll through the output to check for any warnings.

Chkrootkit doesn’t write report other than outputting to the screen by default, but if you wish to automate the checks or to take a look at the findings later, use tee to redirect the printout to a log file.

sudo chkrootkit | sudo tee /var/log/chkrootkit/chkrootkit.log

You can then easily check the log for any warnings.

sudo cat /var/log/chkrootkit/chkrootkit.log | grep -i warning

While chkrootkit can be used to help determine if a machine has been compromised, it shouldn’t be taken as the ‘final word’, use it in together with other scanners to diagnose any possible infections.